In many electrical control systems, there is an undesirable time lag between the instant at which a correction in the output signal is commanded and the instant at which the full correction of the output signal is realized. This occurs, for example, in a control system which includes a gain control circuit and an integrator. In this kind of control system, the gain control circuit receives an input signal and controls its gain to provide a gain controlled signal. The integrator integrates the gain controlled signal to provide an output signal. The output signal is monitored by a threshold detector which in turn controls the gain of the gain control circuit.
If the output signal is outside the limits established by the threshold detector, the threshold detector provides a signal to change the gain of the gain control circuit. Because the integrator performs delaying function, it will fully respond to the change in the gain control signal only after some period of time has elapsed. If the gain of the gain control circuit is changed while the integrator is responding, the loop will be unstable and oscillate. Accordingly, it is necessary to prevent further gain changes of the gain control circuit during this response time, and this significantly slows down the response time of the loop.
It can be seen that two separate problems exist with a control system of this type. First, the integrator inherently has an undesirably slow response. Secondly, during the integrator's relatively long response time, the control system is, in a sense, out of control because it is disabled from making additional gain changes. This problem exists not only with integrators, but with any circuit which has significant delay.